A reducing mill such as a sizer and a stretch reducer is used for rolling a tube so that the tube has a prescribed outer size. Known types of reducing mills include a two-roll reducing mill including a plurality of stands each having two rolls, a three-roll reducing mill, and a four-roll reducing mill.
Such a reducing mill typically includes a plurality of stands disposed along a rolling direction line. Each of the stands includes a plurality of rolls having grooves that define a pass shape. For example, in the three-roll reducing mill, three rolls are disposed at equal intervals around the rolling direction line and shifted by 60° around the rolling direction line from those included in the preceding stand. This is for the purpose of equalizing as much as possible the distribution of radial stress exerted on the outer circumference of a pipe or tube (hereinafter as tube) in the process of rolling.
Each of the stands in the four-roll reducing mill includes four rolls having grooves that define a pass shape. The four rolls are disposed at equal intervals around the rolling direction line and shifted by 45° around the rolling direction line from those in the preceding stand.
In general, the each grooved roll included in each of the stands in the reducing mill has an arch shape in cross section. As shown in FIG. 1, the grooved roll 200 in a three-roll reducing mill has an arc shape of the radius R1 in cross section, which has its center GC on an extension of a segment on the side of the rolling direction line RA that connects the groove bottom GB and the rolling direction line RA. The radius R1 is longer than the distance DB between the groove bottom GB and the rolling direction line RA, so that the distance between the rolling direction line RA and the inner surface of the groove is shortest at DB and longest at DE that connects the rolling direction line RA and the groove edge GE. In short, the groove of the roll 200 has an approximately elliptical arc shape whose minor semi-axis equals DB.
By using the rolls 200, the reduction per stand can be increased. Furthermore, a gap is formed between the outer surface of the tube in the process of rolling and the groove edge GE of the roll 200, and therefore overfilling at the roll gap can be prevented, which can prevent roll edge marks on the outer surface of the tube.
By using the rolls 200, however, large radial stress is exerted on the part of the tube that contacts the bottom of the rolls 200. The distribution of the radial stress during rolling is unequal at the outer circumference of the tube, and the amount of deformation in the radial direction is unequal. The unequal radial deformation results in so-called “polygon formation.” More specifically, as shown in FIG. 2, the shape of the inner surface of the rolled tube is not circular but hexagonal in cross section.
In order to prevent the polygon formation, the distribution of the radial stress exerted on the tube in the process of rolling should be equal. In order to allow the radial stress to be distributed equally, the pass shape profile formed by three rolls should be approximated to a perfect circle. More specifically, the center GC of the arc of the grooved roll 200 should be closer to the rolling direction line RA.
However, when the center GC of the grooved roll 200 is positioned closer to the rolling direction line RA, the gap between the outer circumference of the tube in the process of rolling and the groove edge GE of the roll 200 is reduced. Therefore, overfilling is more easily generated. During rolling, the load exerted on the part of the tube that contacts with the part of the groove surface in the vicinity of the edge GE increases, which is more likely to cause roll edge marks at the part of the tube. More specifically, string-shaped flaws are generated in the longitudinal direction of the tube.
As described above, during rolling the tube, it was difficult to prevent both the polygon formation and the roll edge marks and improve the quality of the tube.
JP 6-238308 A and JP 6-210318 A disclose countermeasures to improve the quality of the tube by rolling with three or more rolls.
A method of rolling with rolls 300 shown in FIG. 3 is disclosed by JP 6-238308 A. The groove bottom 301 of the roll 300 in FIG. 3 has an arc shape in cross sectional whose radius is R1 and its center GC1 is positioned on an extension of a segment on the side of the rolling direction line RA that connects the bottom center GB and the rolling direction line RA. A roll flange portion 302 positioned between the bottom 301 and the groove edge GE is in an arc shape whose radius R2 is larger than the radius R1 and its center GC2 is positioned on an extension on the side of the center GC1 of a segment connecting the end 303 of the bottom 301 and the center GC1. The radius R2 is larger than the distance DB between the bottom center GB in the grooved roll 300 in the preceding stand and the rolling direction line RA. According to the disclosure, by using the rolls 300 for rolling, polygon formation and roll edge marks can be prevented.
However, the center GC1 of the arc of the groove bottom 301 of the roll 300 is positioned on an extension of a segment on the side of rolling direction line RA connecting the bottom center GB and the rolling direction line RA. In short, the grooved roll 300 has an approximately elliptical arc shape whose minor semi-axis equals the distance DB between the rolling direction line RA and the bottom center GB. Therefore, the distribution of radial stress exerted upon the outer circumference of the tube in the process of rolling is not equal and polygon formation could not sufficiently be suppressed.
Meanwhile, JP 6-210318 A discloses a method of rolling using a four-roll reducing mill. According to the disclosure, the radius of curvature of the part of the roll for use in the vicinity of the groove edge is larger than the radius of curvature of the groove bottom, and smaller than the radius of curvature of the groove bottom of the roll in the preceding stand, so that polygon formation can be prevented.
However, the use of such rolls can prevent the polygon formation while roll edge marks are more likely to be caused. Since the distance between the groove edge of the roll and the rolling direction line is shorter than the outer radius of the tube on the stand inlet side, so that overfilling is more likely to be caused, and the load exerted on the part of the tube in contact with the part of the groove surface in the vicinity of the groove edge is large.